Let say start time is 20:00 on 24 hour clock, and you need to add 5 hours to that, but you wish to have wrap around math, so with normal math you state 20:00 + 5:00 = 25:00, but that does not make sense on clock. With modular math, you would say 20:00 + 5:00 mod 24 = 1:00, meaning if you start at 8pm, and you need to add 5 hours to it, you would finish at 1am. In Excel the formula would be =mod(Start + End, 24).

Time is easy to deal with, but when you dealing numbers that wrap around when reaching the modulus of 2 * PI, it a little harder to deal with. For calculation that I am doing, I need to simulate the Mod(x,y) function.